THURSDAY, Feb. 14 (HealthDay News) -- Doctors often urge older patients to get the shingles vaccine because it can prevent or cut the severity of this viral disease. But according to a new study, the vaccine may be less effective in people with untreated depression.

Based on their findings, researchers from the University of California, Los Angeles, suggested the diagnosis and treatment of depression in older people could increase the effectiveness of the shingles vaccine and reduce the risks associated with this painful skin condition. Shingles -- marked by an inflamed rash -- is caused by the reactivation of the virus that causes chickenpox.

The study involved 40 people age 60 or older who were diagnosed with a major depressive disorder. Over the course of two years, the researchers examined the immune response of these patients to the shingles vaccine and compared them to 52 people who were the same age and gender but did not suffer from depression. The participants' immune responses were measured when the study began, then again six weeks later, one year and two years after receiving the vaccine, or an inactive placebo.

The findings, published online Feb. 14 in the journal Clinical Infectious Diseases, revealed that people who had untreated depression had lower immunity to the virus and were less able to respond to the shingles vaccine than those who were not depressed or were taking medications to treat their depression.

The researchers concluded that untreated depression reduced the effectiveness of the shingles vaccine. Antidepressants, however, seemed to "normalize the immune response to the zoster [shingles] vaccine," study leader Dr. Michael Irwin said in a journal news release.

The study authors pointed out that antidepressants increased the effectiveness of the vaccine even when they did not ease a person's symptoms of depression.

While the study found an association between untreated depression and reduced effectiveness of the shingles vaccine, it did not prove a cause-and-effect relationship.

"Efforts are also needed to identify and diagnose depressed elderly patients who might benefit from either a more potent vaccine or a multi-dose vaccination schedule," Irwin noted in the news release.

In addition, more studies are needed to investigate the link between untreated depression and the risk for shingles, the researchers suggested. The link could have far-reaching implications if antidepressants increase the effectiveness of other vaccines, such as the flu shot, for people with depression, they pointed out.

Older adults are at greater risk for shingles, and more than a million new cases occur in the United States every year, according to background information in the news release.